With respect to AB-type monomer of PBO, as early as 1990 there had been a research on the synthesis of 4-(5-amino-6-hydroxy-2-benzoxazol-2-yl)benzoic acid (ABA) and its use in preparing PBO resin by homo-polycondensation in a PPA solvent (Polymer preprints, 1990, 31(2), 681-682). The synthetic route is shown by Scheme (1) and Scheme (2).

However, there was no progress subsequently until Toyo Boseki Kabushiki Kaisha developed a pilot-scale technology to synthesize PBO fibers with AB-type monomer via homo-polycondensation in 2007, which was successively reported in WO 2007032296 A1 in March 2007 and US 20080269455 A1 in October 2008. The scheme (2) completely avoided the release and interference of HCl gas, implemented the equimolar homopolymerization of polycondensation-groups, and thus the polymerization efficiency was greatly improved. However, there exist some problems in scheme (1), for example, ABA prepared by scheme (1) comprises residual DMF which is a polymerization inhibitor and is difficult to achieve polymer-grade quality, and the key starting material 4-amino-6-nitroresorcinol (ANR) for preparing ABA is easy to decompose and difficult to obtain. The purity of ABA prepared by Toyobo and recrystallized using DMF-methanol was only 99%, and the polymerization still need to be carried out by addition of antioxidant stannous chloride and under severe conditions of polymerization temperature at 220° C., total polymerization time in a twin-screw polymerization reactor of 6 h, an intrinsic viscosity [η] of 34 dl/g. The literature (Polymer preprints, 1990, 31 (2), 681-682) disclosed that the polymer with an intrinsic viscosity of up to 12.5 dl/g was obtained by feeding nitrogen under a pressure-reducing condition for 3 h instead of the addition of stannous chloride, dissolving the ABA at 90° C. for 12 h, and polymerizing the monomer at 120-200° C. for 9.5 h. Besides, there are still a lot of other problems in the process of preparing ABA by scheme (1), such as numerous steps, low yield, high price and large usage of the dehydrating agent TSPP, high cost of catalyst Pd/C, and using different organic solvents in each of the three reaction steps of the process. Therefore, the cost of preparing PBO resin by the above method is close to the selling price of PBO fibers, which is lack of practicality.
The inventor has successively designed and synthesized an AB-type PBO monomer methyl 4-(5-amino-6-hydroxybenzoxazol-2-yl)benzoate (MAB), and its key intermediate 4-amino-6-nitro resorcinol hydrochloride (ANR.HCl), and an AB-type PBO monomer ABA which is prepared from methyl 4-(5-nitro-6-hydroxybenzoxazol-2-yl)benzoate (MNB) according to Patent 1 (CN 2006 10155719.8). On the basis of all the researches above-mentioned, a new AB-type monomer named as 4-(5-amino-6-hydroxybenzoxazol-2-yl)benzoic acid carboxy-amino inner salt (ABAS) was synthesized according to Scheme (3). It has been proved that ABA prepared by hydrolyzing MNB and then reducing NBA according to Patent 1(CN 200610155719.8) and ABAS prepared by hydrolyzing MAB and then precipitating the product according to Patent 2 (CN 2006 10155718.3) are tautomers, and ABAS is a carboxy-amino inner salt of ABA.
Compared with other literatures and the patents of Toyobo, the AB-type monomers prepared according to Patent 1 and Patent 2 do not contain DMF which is a polymerization inhibition impurity and have advantages of simple preparation technology and superior economy, but their application results do not improve obviously, mainly because the new AB-type monomers contain too many metal ions or couldn't reach polymer grade (specifically, the monomers prepared according to the methods disclosed in Patent 1 and Patent 2 contain a little of sulfites and metal ions with content of 5000-10000 ppm, and the processes of preparing the monomers result in pollution through waste water containing sulfites), which hinder ABA self-condensation, thereby resulting in obtaining PBO with intrinsic viscosity of only 15 dl/g, which is slightly higher than that of PBO prepared by the literature (Polymer preprints, 1990, 31 (2), 681-682). Therefore, in order to prepare PBO with ultra-high molecular weight, it is particularly important and urgent to prepare a polymer grade AB-type PBO monomer, improve the method of preparing ABA as shown in scheme (3), and develop a method with low pollution and effective removal of metal ions.